The field includes opposed-piston engines in which a combustion chamber is defined between end surfaces of pistons disposed in opposition in the bore of a ported cylinder. More particularly, the field includes opposed-piston engines with combustion chamber constructions that promote complex, turbulent bulk motion in charge air admitted into the cylinder.
A two-stroke cycle engine is an internal combustion engine that completes a cycle of operation with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder for reciprocating movement in opposing directions. Per FIG. 1, an opposed-piston engine includes at least one cylinder 10 with a bore 12 and longitudinally-displaced intake and exhaust ports 14 and 16 machined or formed therein. (In some aspects, when the term “cylinder” is used in this application, it refers also to a cylinder liner.) One or more fuel injectors 17 are secured in injector ports (ports where injectors are positioned) that open through the side surface of the cylinder. Two pistons 20, 22 according to the prior art are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is denominated as the “intake” piston because of its proximity to the intake port 14. Similarly, the piston 22 is denominated as the “exhaust” piston because of its proximity to the exhaust port 16.
Operation of an opposed-piston engine with one or more ported cylinders (cylinders with longitudinally-spaced intake and exhaust ports formed in a sidewall thereof) such as the cylinder 10 is well understood. In this regard, a power stroke commences when, in response to combustion, the opposed pistons move away from respective top center (TC) positions where they are at their innermost positions in the cylinder 10. While moving from TC, the pistons keep their associated ports closed until they approach respective bottom center (BC) positions where they are at their outermost positions in the cylinder. The pistons may move in phase so that the intake and exhaust ports 14, 16 open and close in unison. Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times.
For example, presume the exhaust piston leads the intake piston and the phase offset causes the pistons to move around their BC positions in a sequence in which the exhaust port 16 opens as the exhaust piston 22 moves through BC while the intake port 14 is still closed so that combustion gasses start to flow out of the exhaust port 16. As the pistons continue moving away from each other, the intake piston 20 moves through BC causing the intake port 14 to open while the exhaust port 16 is still open. As the pistons reverse direction, the exhaust port closes first, followed by the intake port.
A compression stroke commences when the pistons reverse direction and move from BC toward TC positions. As the pistons move away from their BC positions their movements are phased such that the port openings overlap to promote scavenging. In scavenging, a charge of pressurized air is forced into the cylinder 10 through the open intake port 14, driving exhaust gasses out of the cylinder through the open exhaust port 16. Typically, the charge of fresh air is swirled as it passes through ramped openings of the intake port 14. With reference to FIG. 1, the swirling motion (or simply, “swirl”) is a generally helical movement of charge air that circulates around the cylinder's longitudinal axis and moves longitudinally through the bore of the cylinder 10. Per FIG. 2, as the pistons 20, 22 continue moving toward TC, the ports close and the swirling charge air remaining in the cylinder is compressed between the end surfaces 20e and 22e. As the pistons near their respective TC locations in the cylinder bore, fuel 40 is injected into the compressed charge air 30, between the end surfaces 20e, 22e of the pistons. As injection continues, the swirling mixture of air and fuel is increasingly compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e. When the mixture reaches an ignition temperature, fuel ignites in the combustion chamber, initiating another power stroke by driving the pistons apart toward their respective BC locations.
The geometries of the intake port openings and the cylinder of an opposed-piston engine provide a very effective platform for generation of a strong bulk fluid motion of the charge air in the form of swirl that promotes both removal of exhaust gasses (scavenging) and the movement of fuel to air (air/fuel mixing). However, charge air motion that is dominated by swirl can produce undesirable effects during combustion. For example, during combustion in a cylindrical combustion chamber defined between flat piston end surfaces, swirl pushes the flame toward the cylinder bore, causing heat loss to the (relatively) cooler cylinder wall. The higher velocity vectors of swirl occur near the cylinder wall, which provides the worst scenario for heat losses: high temperature gas with velocity that transfers heat to the cylinder wall and lowers the thermal efficiency of the engine. The peripheries of the piston end surfaces also receive a relatively high heat load, which causes formation of a solid residue of oil coke that remains in the piston/cylinder interface and in the ring grooves when lubricating oil breaks down at high engine temperatures.
Accordingly, it is desirable to maintain the benefits provided by swirl while mitigating its undesirable effects as combustion begins. At the same time, it is desirable to continue to promote turbulence in the charge air motion in order to encourage a homogeneous mixture of fuel and air, which in turn, produces more complete and more uniform ignition than would otherwise occur.
These advantages have been achieved in two-stroke opposed-piston engines by provision of shapes in the opposing end surfaces of the pistons that generate additional components of bulk air turbulence in the combustion chamber. In this regard, certain opposed-piston combustion chamber constructions include surfaces that generate squish flow from the periphery of the combustion chamber in a radial direction of the cylinder toward the cylinder's axis. In some aspects, squish flow can be inwardly directed as when a high pressure region at the peripheries of the piston end surfaces causes charge air to flow to a lower-pressure region generated by a bowl formed in at least one piston end surface.
U.S. Pat. No. 1,523,453 describes a pair of opposed pistons having depressions formed in their heads which form a pear-shaped combustion chamber when the pistons are adjacent each other. The larger end of the chamber is substantially closed and the smaller end is open to permit injection of fuel into the chamber by an injection valve in the cylinder wall. The pear-like shape of the combustion chamber produces two desirable effects. First, it accommodates a single, cone-shaped spray of injected fuel that is proximate to, but does not contact, the end surfaces of the pistons. Second, as the pistons reciprocate, the shape moves air in such a manner as to promote good air/fuel mixing and effective combustion.
A number of recently-disclosed opposed-piston designs have been directed to generation of tumble in bulk motion of charge air. For example, related U.S. application Ser. No. 13/066,589 describes formation of an ellipsoidally-shaped combustion chamber between projecting curved ridges in the adjacent end surfaces of opposed pistons. The curved ridges are identical, but mutually rotated by 180°. The end surfaces interact with swirl and squish flows to produce tumble at the narrow ends of the combustion chamber, near the bore surface of the cylinder. The wider central portion of the combustion chamber preserves swirl. Related application Ser. No. 13/843,686 describes an improvement to this mutually-inverted ridge configuration in which the central portion of the combustion chamber has a pronounced spherical aspect that preserves more swirl than the mainly ellipsoidal shape. An ellipsoidally-shaped combustion chamber formed between opposed pistons having non-identical, but complementary end surface shapes is described in the related PCT application. In this construction, a concave bowl is formed in one end surface. The opposing end surface has a convex projection in which a bilaterally-tapered, diametrical cleft is formed between mirrored, continuously curved ridges. When the end surfaces are adjacent, the convex projection is received in the concave bowl and the combustion chamber is defined principally by the cleft. Bordering squish regions are formed on either side of the chamber by opposing convex/concave end surface portions.
In related application Ser. No. 14/026,931, a combustion chamber for an opposed-piston engine has a bowl/mirrored ridge construction that distributes the combustion chamber volume between a central, largely spherical (or spheroidal) volume which holds most of the heat of combustion and lateral spray clearance channels that are mutually aligned along an injection axis and disposed on respective sides of the central volume. The bowl construction is provided on the end surface of one piston of a pair of opposed pistons and the mirrored ridge construction is provided on the end surface of the other piston of the pair.
The combustion chamber shapes described in the related applications cause swirling charge air to interact with contoured piston end surfaces to produce turbulent bulk air motion that includes elements of swirl, squish, and tumble. The combustion chambers have elongated shapes with tapered ends that cause the air motion elements to be oriented with respect to a direction in which fuel is injected. In many of these cases, fuel is injected from two diametrically or near diametrically opposed injectors with nozzles positioned at the tapered ends. Each injector injects fuel through multiple nozzle holes, which produces a cone-like spray pattern with multiple, diverging plumes. It is desirable that the spray patterns interact with the turbulently-moving charge air in such a way as to create an optimal mixture of air and fuel for effective combustion. For this and other objectives, the shape of the combustion chamber should promote the following desirable results.
The combustion chamber shape of an opposed-piston engine should minimize the interaction of the plumes in each injected spray pattern, as well as interactions between the opposing spray patterns as they approach each other. Air/fuel mixing should be maximized for the fuel injected by each injector. The residence time of flame on the piston end surfaces should be minimized. The elements of turbulent air motion should be felt by the multiple plumes issuing from each injector.